At present, LED lights are generally driven by a switch mode power supply. Commercial power is converted into low-voltage direct current by a high-frequency switch mode power supply so as to drive LEDs to emit light. The LEDs driven by the switch mode power supply have the disadvantages of complicated circuit, many components, high cost, low reliability and large size. The introduction of the high-frequency switch mode power supply will generate electromagnetic interference and will directly influence the service life of the whole LED light. Many failures of the LED light are resulted from the power supply failure.
To solve this problem, there is a linear constant-current LED light circuit without a switch mode power supply. The voltage of an individual LED is generally about 3.2 V, which is far less than the voltage of the commercial power. Therefore, multiple LEDs are connected in series to form a high-voltage LED string so that the total voltage is close to the voltage of the commercial power, and the LEDs are allowed to operate by a current limiting circuit. As shown in FIG. 1, multiple LEDs are connected in series to form a high-voltage LED string so that the total voltage is close to the voltage of the commercial power, where the voltage of an individual LED is 3V and n individual LEDs are connected in series to form the high-voltage LED string having a voltage of (3 n)V. When the input voltage is less than Vd, the LEDs are in the off state and will not emit light; and, when the input voltage is greater than Vd, the LEDs are powered on to emit light. However, the operating voltage of the LEDs in this circuit is fluctuant, and the operating current is discontinuous. Accordingly, this circuit has the disadvantages of small current turn-on angle, obvious stroboflash of LEDs, low electrical efficiency, low harmonic distortion and low performance index.
Therefore, how to provide an LED light having long service life, no stroboflash and high power factor becomes a technical problem to be urgently solved by those skilled in the art.